Currently, external preparations containing lidocaine having a local anesthetic effect are marketed for the purpose of alleviating, for example, nerve pain caused by herpes zoster, and development of such external preparations containing lidocaine is being advanced.
For example, Japanese Unexamined Patent Application Publication No. Hei 4-305523 (PTL 1) discloses a hydrogel patch for external use comprising an adhesive gel base containing lidocaine, wherein the adhesive gel base comprises a water-soluble high molecular weight substance, water and a water-retaining agent as essential ingredients. PTL 1 also states that the pH of the gel base is preferably in the range from 5 to 9, and that the gel base may comprises a conventional absorbing agent from the viewpoints of skin irritancy and stability. PTL 1 shows oleic acid and the like as examples of the absorbing agent.
Moreover, International Application Japanese-Phase Publication No. 2001-503035 (PTL 2) discloses a local anesthetic for external use containing: an active ingredient such as lidocaine; water; a lower alcohol such as ethanol; and a percutaneous absorption accelerator. PTL 2 shows fatty acids such as oleic acid as examples of the percutaneous absorption accelerator, and states that the pH of the local anesthetic for external use is preferably in the range from 6.0 to 8.5.